Bob, are you clicking the button at the top (and bottom) of the thread page in order to open the reply message box?? Where is this "respond" link located on the page? I suppose this is where a screenshot would be handy, showing not the invalid error message, but the page with the "respond" link or button that you're clicking. Please include the url address at the top in the screenshot

edit to add: I get that https://forum.lowcarber.org/newreply.php url when I preview my reply before submitting. However, there's no "respond" button or link, just "Submit Reply" and "Preview Post".